The transconductance, gate to source voltage vs drain current, of Gallium arsenide (GaAs) transistors changes significant when operated from d.C. to 100 KHZ. GaAs transistors are necessary to improve the high frequency operation of existing silicon transistor circuits, but the GaAs transistor circuits must maintain the same low frequency accuracy of silicon transistor circuits.
Field Effect Transistors (FETS) are voltage controlled current sources, where the drain current (I.sub.D) is the current source and the gate to source voltage (V.sub.GS) is the controlling voltage. For GaAs FETS, the transconductance (I.sub.D versus V.sub.GS) of the FET changes significantly at low operating frequencies. This is for frequencies from D.C. to approximately 100 KHZ. For high frequency changes, frequencies greater than 100 KHZ, the gain does not change significantly.
In GaAs FETs, when the drain current is held constant, the gate to source voltage will change if the drain to gate voltage of the FET changes at low frequencies. Gate to source follower circuits, such as voltage buffers and drivers, operate at a relatively constant drain current. When the gate to source voltage is held constant, the drain current will change relative to a change in drain to gate voltage at low frequency. FET current sources operate with a constant gate to source voltage.
For other circuits using GaAs FETs, all the bias conditions change with operating frequency. Amplifiers and differential amplifiers are two examples. When the bias conditions change at low frequencies, the transconductance of a circuit using GaAs FETs will change. All the circuits described above will have a change of transconductance when operated at low frequencies. Comparators and logic gates are implemented with some or all the circuits above. For logic gates and comparators, the change in transconductance causes threshold accuracy errors which relate to timing errors. The error is not a simple offset, gain, or nonlinearity error, the error is frequency dependent. For an input that has a random change in operating frequency, the error cannot be predicted. This low frequency dependent transconductance error is know in the industry as hysteresis.